convert date to MSExcel format?

I am new to Java and am experimenting with the language. I am looking a way to convert a user-inputted date and convert it to the MSexcel format. So 1998/07/05 is 35981. Any clue on how to do this?
This is what I have so far

day = input.nextInt();
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:909)
at java.util.Scanner.next(Scanner.java:1530)
at java.util.Scanner.nextInt(Scanner.java:2160)
at java.util.Scanner.nextInt(Scanner.java:2119)
at A1Q1.main(A1Q1.java:65)

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